New guy here... quick question for the pros

[smogrods]

hey every one ive been lurking for a few weeks now and i came across something weird on my c-10.

I bought this 1970 c-10 lwb in oct

and have been tinkering on it getting it to a reliable driving state since. ive been rewiring the entire truck.

well when i was under it today i noticed it has 2 fuel tanks one in the cab and one under the bed.. as well as a selector switch that i only assume is a tank selector on the floor.

where is the fuel filler for the under bed tank?
please see my pics.

here is the tank selector switch (i think)


the cab tank

some shots of the under bed tank


some shots of the side of my truck.